Even if you live and work in Canada, you may still have to file a US tax return. If you have an American parent, spend a lot of time in the US, or are a dual citizen, as Kim Parlee reports, you may have to report to the IRS.
Despite that, you could still owe taxes to the United States if you have connections to the US that make you a US person from the perspective of the Internal Revenue Service.
For example, for tax purposes a US person could be summoned with a joint US-Canadian citizenship, someone whose parent is a US citizen, or even someone who just works a certain number of days in the United States.
If you fall into one of those categories, or any others, you should find out if you need to file and pay US income tax.
You can get more information by going to the website irs.gov and talk to a financial advisor or talk to a US tax specialist.
Bottom line-- don't neglect it.
Ignoring the fact that you may owe US taxes can complicate matters and cost you.